A common feature of such electrochemical cells is the interface between the ceramic surface of the electrolyte and the ceramic/metallic/composite electrode must meet two vital criteria, viz. a) mechanical integrity; and b) electronic and/or ionic contact.
The requirement to mechanical integrity is essentially based on the fact that the electrode must not let go of the electrolyte. Typically, electrodes can be applied onto electrolytes by way of wet-ceramic processes followed by a sintering, or by way of rather advanced methods, such as CVD, laser ablation etc at an increased temperature. The two materials are joined at a temperature referred to as the process temperature below. It is often advantageous to use a relatively high process temperature because such a temperature results in a good mechanical adhesion through a reaction between the two materials.
When the unit is cooled or heated, mechanical stresses arise between the electrode and the electrolyte irrespective of the applying method unless said electrode and said electrolyte present exactly the same thermal coefficient of expansion. These stresses escalate at the same rate as the temperature differs from the process temperature, and therefore these stresses are strongest at low temperatures, such as at room temperature.
Another type of mechanical stresses between the electrode and the electrolyte is caused by volumetric changes of specific metals/metal oxides in connection with a change in the oxygen partial pressure pO2 of the atmosphere at the operating temperature. The volume of many metal oxides is changed either by releasing or absorbing oxygen concurrently with changes in the oxygen partial pressure, cf. for instance CeO2 expanding reversibly at reduction. Another example is NiO, which is converted into Ni at reduction. The Ni can subsequently be oxidized into NiO.
The requirement to electronic and/or ionic contacts is substantially based on the fact that a charge transfer must be allowed in the electrode or at the interface to the electrolyte, said charge transfer involving ions and electrons. The latter charge transfer can be carried out on the surface of specific types of electrodes, viz. mixed conductors, or along portions of the three-phase limit formed by the interface between the electron conductor, viz. the electrode, the ion conductor, viz. the electrolyte or the ion conductor in the electrode, and the gas phase. In order to optimize the operation of the electrode it is essential to minimize possible losses in connection with the above electrode process. These losses can be electrochemically measured as an electrode overpotential (V), or as an impedance (Ω) called the polarisation resistance of the electrode.
A first cause of loss is formation of nonconductive reaction products at the interface.
Another cause of loss is reduction of the three-phase limit or the electrode face by way of a reduction of the porosity of the electrode.
A third cause is found in loss of conductivity due to a mixing of the two materials, viz. reactivity or reciprocal solubility.
An example of the latter is formation of poorly conducting zirkonates between (La,Sr)MnO3 (=LSM) and Y-stabilized ZrO2 (=YSZ).
Yet another example of the above is reciprocal diffusion and formation of porosities at the interface between Gd-doped CeO2 and YSZ.
Thus it is essential to minimize or completely avoid reactions and formation of reaction products at the interface between the electrode and the electrolyte in order to minimize the polarization resistance. Such formations of reaction products and poor contact surfaces are typically avoided at high temperatures, and accordingly a low process temperature is preferred.
Thus it appears that high applying temperatures are typically required in order to obtain a good mechanical interface. However, in addition it appears that these high temperatures may cause formation of reaction products at the interface, said reaction products reducing the efficiency of the electrode.
The electrodes and the electrolyte can be sintered simultaneously, viz. sintered at the same time, provided such a procedure is allowed in view of the reactivity. As the electrolyte must be tightly sintered, the process temperature is often high, i.e. higer than 1250° C. for YSZ, and only very few of the known electrode materials are suited for a sintering simultaneously with the electrolyte at such high temperatures.
In addition, it is known to increase the mechanical strength of the interface between the electrode and the electrolyte by roughening the usually smooth surface of the electrolyte either by way of a corrugation as described in EP A1 0615299 or by way of a sintering thereon of electrolyte particles. Such a procedure is particularly advantageous in resulting in an increase of the interface and the three-phase limit and consequently of the efficiency of the electrode. Such a roughness must typically exceed the particle size of the electrode layer sintered thereon in order to ensure a substantial anchoring.
However, it is difficult to corrugate a very thin electrolyte, viz. an electrolyte of a thickness of 5 to 25 μm, without causing errors, such as through errors and porosities. Consequently, it is difficult to sinter electrolyte particles onto a tight electrolyte because typically the electrolyte is not sinteringly active. In addition, it is difficult to obtain a tight electrolyte by way of sintering while electrolyte particles are simultaneously sintered thereon, said electrolyte particles projecting from the surface. Typically, such particles cause through errors and porosities.
Another possibility is to increase the sintering temperature until a suitable mechanical anchoring of the electrode material has been obtained provided a predetermined limitation of the efficiency of the electrode is simultaneously accepted.
A third possibility is to apply a third material in form of a continuous, optionally porous membrane on for instance the electrolyte in order to prevent a physical contact with the reactive electrode. However, such a membrane must meet the same requirements with respect to mechanical integrity and electronic and/or ionic contact relative to both the electrode and the electrolyte.
Finally, it is known to laminate a layer of particles of the composition of the electrolyte between the electrolyte and the electrode prior to the first sintering, cf. JP 10012247. However, such a procedure involves a high risk of damaging the electrolyte layer, which is typically of a thickness of 2 to 15 μm corresponding to the dimension of efficient anchoring particles.